The present invention relates to a hardness testing apparatus operating with a preload and a main load or metering load, wherein the indentor moves relative to a reference plane, which normally abuts on the workpiece to be tested, said hardness testing apparatus being provided with a safety stop device.
Hardness testing apparatuses, acting with a preload and a main load are known and they operate with a very high applied load, for example up to 3.000 Kg, wherein the applied load is transmitted by a thrust member apt to furnish a thrust, coaxial or parallel to the indentor axis, for example the rod of an hydraulic jack.
Because of the high applied load, the operation of said hardness testing apparatuses may become dangerous in some cases, such as a failed truing of the workpiece to be tested or a great inclination of the same workpiece, so that the indentor tip projects more than the usual rate beyond the reference plane, causing failures to the members stressed by the main load.
Another dangerous case is when the apparatus can harm the workers, for example in case of a careless insertion of a hand between the indentor and the workpiece to be tested.
Devices able to verify the penetrating rate of the indentor are already known in the prior art, but they are designed to ascertain if the workpiece hardness is within the required limits.
The need to test the real hardness of the workpiece through the preload and main load system, has always required the application of the main load, so that said known devices are not employable as safety devices able to prevent the application of the main load, in the event that a hand is interposed between the workpiece to be tested and the indentor, or in the event that the workpiece to be tested does not interfere with the indentor while interferring with the reference plane.
Examples of said known devices are set forth in U.S. Pat. No. 2,858,696 and U.K. Pat. No. 1,245,295.
U.K. Pat. No. 1,183,277 teaches a safety device for a manually controlled instrument able to signal that the testing cycle must be stopped when the preload is disturbed by the anticipated application of the main load. In any case, however, the stop of the cycle takes place only after the main load has been applied.
U.S. Pat. No. 4,059,990 teaches a hardness testing apparatus that seems to operate with a high main load, as the indentor is moved by a double-acting cylinder.
A control fluid flow, such as air, is more or less throttled proportionally to the workpiece hardness and a gauge responsive to this throttling causes the starting of an alarm or the ejection of the workpiece being tested, when the hardness amount is out of the predetermined limits.
Even in this case, however, the device operates when the main load has been applied.
The U.S. Pat. No. 3,934,463 teaches a hardness testing apparatus which does not operate according to the preload and main load system, but it operates in such a way that the indentor penetrates into the workpiece at a predetermined rate and the hardness is tested with reference to the load which has been applied on the indentor. Even in this event, a dangerous applied load acts on the indentor and on the reference plane.